In a wireless communication system, MIMO techniques are widely adopted to reach higher system capacity using multiple antennas at both a transmitter and a receiver. In general, there are two categories of MIMO systems: diversity multiplexing and spatial multiplexing. The Alamouti space-time block code (STBC) can achieve full diversity for two transmit antennas with one spatial stream. However, in most cases, there are more than two transmit antennas with multiple spatial streams in a MIMO system. Therefore, combining STBC with spatial multiplexing becomes a critical issue in designing a MIMO system.
STBC is an optional feature for open loop architectures. STBC can achieve full diversity without knowledge of the channel state information (CSI) at the transmitter. For example, for consecutive symbols S1 and S2, the Alamouti STBC encoder is represented by a 2×2 block matrix as:
                    [                                                            S                1                                                                    -                                  S                  2                  *                                                                                                        S                2                                                                    S                1                *                                                    ]                            (        1        )            
where S is complex and S* is conjugate of S, and elements in the same row will be transmitted from the same antenna and each column of elements will be transmitted at the same time. As such, at time 1 antenna 1 transmits S1, and antenna 2 transmits S2, etc.
As discussed, and shown by relation (1) above, the Alamouti STBC is suitable for two transmit antennas with one spatial data stream. In order to use STBC in a system with a higher number of transmit antennas and multiple data streams, a conventional approach attempts to combine STBC with spatial multiplexing. For a number (Nt) of transmit antennas equal to twice a number (Nss) of data streams, the mapping of the data streams to the transmit antennas is straightforward because each data stream can be mapped into two transmit antennas using a 2×2 Alamouti STBC encoding block. For other cases, however, the conventional approach leads to unequal STBC protection, posing significant problems.
For example, as shown in transmitter 100 of FIG. 1, with Nt=3 and Nss=2, an STBC encoder 102 according to the conventional approach maps a first data stream (Stream 1: S11 S12 S13 S14 S15 S16 S17 S18) onto transmitter antennas Tx0 and Tx1 using STBC encoding, and a second data stream (Stream 2: S21 S22 S23 S24 S25 S26 S27 S28) is mapped onto antenna Tx2 without the STBC protection. Since the first stream has diversity gain due to the use of STBC encoding, and the second stream does not, the received signals at a receiver will operate at different SNRs. However, both data streams support the same modulation and coding rate, wherein the stream without STBC protection will dominate the performance. In this case, the performance enhancement from the use of such conventional STBC encoding is very limited because any errors from any one of the streams will lead to an error packet.
There is, therefore, a need for a STBC-based transmission method with equal protection on all data streams such that all received streams operate at the same SNR. There is also a need for a method to combine STBC encoding and spatial multiplexing for performance enhancements in high throughput WLANs.